Inhibition of the specific binding of (3H) kainic acid was studied in membranes isolated from rat and human cerebellum. Whereas the Hill coefficient for unlabelled kainate is 1.0, the other compounds exhibit coefficents near 0.5, suggestive of cooperative interactions at the receptor. In human postmortem brains, samples from patients who died with Huntington's disease, show decreased (3H) kainic acid receptor binding in the caudate nucleus, putamen and frontal, cortex, but not insular or temporal cortex, hippocampus or cerebellum. High affinity binding is profoundly reduced in the caudate nucleus and putamen. These regions of brain are most affected in Huntington's disease.